1. Field of the Invention
This invention relates to plate type dampers, and more particularly, to such dampers for controlling the flow of hot gases in conduits such as breeching ports of various types of furnaces, for example.
2. Description of the Prior Art
In certain high temperature furnaces, such as multiple hearth furnaces of the Herreshoff type, a plurality of hearths, or hearth chambers, are vertically aligned and an elongated shaft extends up through the center of the furnace, passing through the floor of each hearth. Rabble arms are secured to the shaft and extend radially outwardly therefrom over each hearth floor. These arms are provided with rabble teeth which extend down into material being processed on the hearths. As the center shaft rotates, the rabble arms move over the material on the hearth floor causing the teeth to plow through it. Depending on the inclination of the teeth, the material will be moved radially inwardly toward the center shaft or outwardly therefrom. Drop holes are provided in each hearth floor either in toward the center shaft or out toward the furnace wall so that, as the material completes its movement over a hearth floor, it will drop down into the next lower hearth and move across that hearth in the direction opposite to that in the next adjacent upper hearth. Thus, the material is caused to move slowly in serpentine fashion through the furnace.
Because these furnaces permit close control of individual hearth atmospheres and temperatures, it is possible to perform very delicate operations such as the regeneration of spent bone char from the refining of cane and beet sugar, for example, while numerous other thermal treatment processes can also be performed. Thus, U.S. Pat. No. 3,153,633 to Von Dreusche discloses the use of a multiple hearth furnace for the regeneration of certain granular activated carbon absorbing agents.
In most furnaces of the aforementioned type, many of the hearths are directly fired and the heat requirement of each fired hearth is adjusted by varying the flow of air and fuel and the air-to-fuel ratio to each fired hearth. However, in some cases, it is necessary to fire the hearth chambers indirectly by supplying hot gases from an external combustion chamber and the flow of these gases to respective hearth chambers is controlled by dampers.
Butterfly dampers associated with various furnace hearth chambers have been designed for service at temperatures well below 2000.degree. F. and such dampers have been constructed of high temperature nickel-chrome alloys and have been air cooled, water cooled or not cooled at all. However, operating and maintenance problems have been encountered with these butterfly type alloy dampers. Thus, on a recent commercially operated furnace of the type described, wherein it has been necessary to provide external combustion chambers indirectly to supply heat to each hearth to reduce cost and to obviate the possibility of flame impingement on the material being processed, two external combustion chambers were used with breechings to each of three hearth chambers. To control the flow of gases to these hearths, an extremely high temperature damper was required since the temperature attained at the damper positions was of the order of about 2300.degree. F. to about 2800.degree. F.
Water cooling of alloy dampers was considered, but this has the disadvantages of being extremely expensive, complicated and potentially dangerous in the event of water leakage. Refractory guillotine type dampers were also considered, but these are bulky, heavy, expensive, and difficult to maintain when airtightness is a requirement. Thus, these considerations were rejected.